I managed to snag a PCjr from a flea market earlier today and, a little to my surprise, it booted right up. Came with the IBM 4863 monitor and a spare keyboard as well.
Not having worked with or seen a PCjr up close ever (I have 2 5150's and a Compaq portable), I started the exploration to see what's in this box and whether or not everything works. This forum was great to get me started!
Based on what I've seen in this system, though, it looks like I have a Racore Radrive II on this machine. I started to test the components in this expansion unit, and the floppy drive works fine, and so does the real-time clock (after I replaced the battery). However, the memory is a different story. When booting up, the PCjr counts to 128K and then boots. There is, what looks like a memory card in the Radrive II expansion box, fully outfitted with RAM chips and labeled A2101-000. Doing some digging, this should mean I have 512Kb of memory on this thing..
When I look with memory dump tools, contents from 00000H to 1FFFFH are stable, but beyond 20000H it's a pile of junk that seems to have bytes flip constantly. So I'm guessing that something is wrong.

Does anyone have any insights on what could be going on? I did a few simple tests, and it looks like the board is getting power, but I didn't go any further to look at the data lines, etc. I did remove the sidecar and refitted it, based on some posts from people saying that it could be a little iffy. No change there.. Pic of the board attached.
Any help is appreciated!